In 1996, more than 75 million people worldwide used cellular telephones. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
This competition has also led to rapid and sweeping innovations in cellular telephone technology. Analog cellular systems are now competing with digital cellular systems. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the increased number of cellular base stations has resulted in increased infrastructure costs. To offset this increased cost, cellular service providers are eager to implement any innovations that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, as well as the number of subscribers that the cellular system can service.
Much of this innovation has focused on service quality improvements, such as expanded digital PCS services or smaller and lighter cellular phone handsets having a longer battery life, or equipment cost reduction, such as smaller, cheaper, more reliable transceivers for the cellular base stations. However, there has been only limited innovation directed to reducing the operating costs of a cellular system.
Every cellular base station has a transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computer equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the transmitter power amplifier is highly linear, especially for a signal whose envelope changes in time over such a wide range, such as in CDMA or multi-carrier operations. One of the techniques used in the design of highly linear amplifiers is known as the feedforward technique. In the prior art, it is based on two-loop designs. Typically, the first loop isolates the error signal produced by intermodulation distortion in the power amplifier and the second loop subtracts the error signal from the power amplifier output. However, the two-loop designs contain a high number of components and require delay lines in the power amplifier outputs that consume a large amount of power and greatly reduce the efficiency of the transmitter. The need for delay lines is due in part to the fact that the prior art amplifiers are, by and large, analog designs that are unable to buffer signals.
There is therefore a need in the art for improved cellular systems that are less expensive to operate. In particular, there is a need in the art for improved transmitter power amplifiers based on simpler feedforward designs that do not require the use of a delay line in the power amplifier output. There is a further need for feedforward transmitter power amplifiers that are implemented using more reliable digital design techniques, particularly in CDMA systems.